I guess it is the same like Karin's pants warmer, 12V 50 Watts, that is some 4A and it comes from the 12V battery not the HV. Ours is dead when charging except when I put the key in and at least switch to ACC.
As far as I can see our 2010 i-MiEV does not charge the 12V. That is why I have got us a 12V charger, 4/6 A that is 6A eff or seen by Murphy and 4A arith(metically) for the heater actually. Our 12V battery gets only charged when the dashboard says "ready" and when charging it never gets ready. That may be different for later i-MiEVs.
The i-MiEV has got an extremly tiny 12V battery. We do not need it to start an engine after all. That is why it is good to have a means to charge it, just in case. We do not need a big jumper cable but the thin plug and cable from cigarette liter to cigarette liter will do nicely for us. It is more fool proof the the jumper cable too.